| Daquan area is located in the western section of Middle Qilian Mountain.Various geologic bodies and tectonic units in the western Qilian orogenic belt have been through strong multi-phase metamorphism.The oldest strata outcropped in the area is the Beidahe rock group which is formed in Paleoproterozoic,suffered multi-phase metamorphism.This rock group belongs to the batholith of western Qilian orogenic belt,the degree of metamorphism is deep,strong tectonical deformation,and widely developed penetrative structural bedding,thus,the original-rock characteristic and original petrofabric were lost or unable to be recognized,showing strata but no sequence,prevented the recovery for the original stratigraphic sequence.In this study,according to the lithology and rock association characteristics,combining with previous datum and regional correlation,we divided the Beidahe Group into b and c formations,with a fault contact.This study focus on the b formation,discuss the rock association characteristics,geochemical characteristics,and try to recover the original rock and tectonic setting,provided important data to the research on the batholith of Western Qilian Mountain.The result showing that the Beidahe Group b formation consists of two-mica schist,garnet two-mica schist,and two-mica quartz schist,with minor gneiss,amphibolite,quartzite,granulite and marble.The metamorphic volcanics in b formation are basic,?=1.50-1.92,calc-alkali rock,with lower Ti O2?Fe2O3?P2O5 and higher Al2O3,Si O2 unsaturation,aluminum saturation;The distribution of trace elements showing slightly difference,overall display strongly incompatible element enrichment characteristics,rich in La,Nd,Sm and depletion in Th,Nb,P,Ti;The abundance of rare earth elements is relatively high,the rocks are rich in light rare earth elements and depletion in heavy rare earth elements,and the heavy rare earth fractionation is not obvious.The metamorphism belongs to high green schist facies to low amphibolite facies.The original rock restoration is mainly a set of detrital rock(sand mudstone construction)-carbonate rock-(basic)volcanic rock.The detrital rock has residual rhythmic stratification,showing the characteristics of flysch macroscopically;the basic volcanic rocks has properties of the intrapleural rift environment,showing a tectonic setting of ocean bed or mid-ocean ridge,indicating that the palaeoproterozoic sedimentary basins are gradually transformed from the active semi-deep-sea environment to the stable shallow sea environment,belongs to shallow-semi-deep sea transition deposit. |
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